The invention relates to a process for the continuous partial crystallization of a liquid mixture, in which this mixture is led in its entirety through a cascade, i.e., a plurality of cooling sections connected in series. The first section of the cascade has a temperature at which a part of the mixture crystallizes and the temperature of each subsequent section is lower than that of the preceding one. The last section has a temperature such that the whole of the liquid mixture is not solidified. The crystals formed in the sections are kept suspended in the liquid and are led through the cascade together with and in the same direction as the liquid.
Furthermore, the invention relates to a process for the separation of a liquid mixture, whereby from this mixture a crystal suspension is produced.
Finally, the invention relates to a device for carrying out the said processes.
By a liquid mixture is meant a mixture which is liquid under operating conditions. It may consist of a number of liquid components, but it may also contain one or more dissolved solid components. The invention is, however, meant in particular for the separation of mixtures of liquid organic compounds, although it is not restricted to such mixtures.
In the process industry the concentration or isolation of one or more components from a liquid mixture plays an important part. A crystallization process can be used for this purpose, the first step of which consists of the partial crystallization of the liquid mixture to be separated. Subsequently, the formed crystals can be separated from the resulting crystal suspension and/or the suspension or crystal mass can be subjected to a further purification treatment.
In the partial crystallization of a liquid mixture it is desirable to obtain relatively pure crystals, i.e., crystals consisting of the component or components to be separated with only a low content of impurities. These crystals must be of such a size that they can easily be separated from a suspension by mechanical means e.g. by filtering, pressing, or centrifuging. Furthermore, rapid growth of the crystals is desired in order to avoid a long residence time and a large, and thus expensive, crystallizer.
When crystals are formed, sponge-like conglomerates of relatively small crystals often develop, which contain liquid impurities as inclusions. When a mixture of substances with related molecular structures, e.g. isomers, is separated, molecules of the one substance can be trapped in crystals of the other substance, resulting in the formation of impure crystals. These phenomena are partially kinetic effects, i.e. they occur in particular at high linear crystallization rates. Furthermore they are promoted by a high concentration of impurities. In order to obtain pure crystals, it is therefore desirable to use low linear crystallization rates and to make the crystallization take place close to equilibrium. If large differences in concentration exist, and more generally under circumstances strongly deviating from equilibrium, strong nucleation takes place, leading to the formation of a large number of very small crystals. In order to prevent nucleation as much as possible, strong supersaturation and/or supercooling are to be avoided.
Purifying crystals by washing with a liquid influences mainly the surface of the cyrstals and has little or no effect on the core. Thus, in the common circumstance where the composition of the crystals is not homogenous, i.e. the concentration of the impurities depends on the location in the crystal, it is recommended that the core be as pure as possible and that any impurities be mainly concentrated on the surface.
It has turned out that the above mentioned requirements can be at least partly met by effecting partial crystallization of a liquid mixture in parallel flow viz. according to the process mentioned in the preamble.
A device for carrying out this process is the cooled disc crystallizer commerically available from Messrs. Stork. This crystallizer consists of an oblong trough which is divided into a plurality of interconnected sections by parallel vertical partitions. These partitions do not span the whole cross section of the trough. On one side of the trough, the liquid mixture to be separated is introduced. It passes successively through all sections leaving the trough on the other side. Each section has a rotating disc ensuring a good stirring and preventing coagulation on the walls. Furthermore, cooling means are present allowing control of the temperature profile over the length of the trough.
The Stork crystallizer provides an acceptable compromise in that it can provide relatively pure crystals of a large and uniform diameter while using a relatively high crystal growth rate. This crystallizer has, however, a serious limitation: crystal suspensions can only be prepared with a maximum crystal volume friction of 0.3-0.4, as suspensions with a higher crystal fraction cannot be effectively stirred and circulated. It is true one could make a Stork crystallizer function in such a way that it yields a crystal suspension with a crystal fraction of e.g. 0.35, which crystal suspension is then led to a mechanical separation device after which the liquid separated in this device is fed to a second Stork-crystallizer. Besides the technical complication, this array has the disadvantage that the liquid adhering to the separated crystals does not participate--at least directly--in the further processing. Also, in the second crystallizer strong nucleation occurs due to the high concentration of the impurities. Because of this, small crystals are formed in the second crystallizer which are not easy to separate.
U.S. Pat. No. 3,197,524 describes a process and device for the recovery and purification of naphthalene by fractional crystallization. The liquid mixture to be separated is led through a cascade of cooling sections; the temperature in each subsequent section is lower than that in the preceding one. The mixture of liquid and crystals leaving the last section is separated in a centrifuge, after which part of the separated liquid is returned to some preceding sections, but not to the last one from which it was removed.
The object of the invention is to provide a process for the continuous partial crystallization in parallel flow of a liquid mixture which process does not show the disadvantages of the known processes.